This invention relates generally to oligonucleotide synthesis and, more particularly, to methods of synthesizing oligonucleotides having random codons using individual monomers.
The speed and availability of automated nucleic acid synthesis has led to rapid technological advances in biological research. For example, the availability of synthetic primers for sequencing has permitted researchers to decrease their time and labor involved in sequencing a particular nucleic acid by approximately sixty percent. Another technology which is facilitated by synthetic oligonucleotides is the polymerase chain reaction (PCR). This technique, which involves the exponential amplification of sequences between two synthetic primers, offers unprecedented detection levels and permits genetic manipulation of the amplified sequence. Further, the availability of synthetic primers allows a variety of genetic manipulations to be performed with relatively simple procedures, including site-specific mutagenesis and the custom design of genetic vectors.
Sequences to be cloned are also routinely modified with synthetic oligonucleotides. The modifications of either vector or insert sequence can range from the addition of a simple sequence encoding a restriction enzyme site to more complicated schemes involving modifying the translation product of the cloned sequence with a specific peptide or a variety of peptide sequences. Thus, these technological advances associated with synthetic oligonucleotides has afforded researchers many opportunities to study diverse biological phenomenon in greater detail and with greater speed and accuracy.
Oligonucleotide synthesis proceeds via linear coupling of individual monomers in a stepwise reaction. The reactions are generally performed on a solid phase support by first coupling the 3' end of the first monomer to the support. The second monomer is added to the 5' end of the first monomer in a condensation reaction to yield a dinucleotide coupled to the solid support. At the end of each coupling reaction, the by-products and unreacted, free monomers are washed away so that the starting material for the next round of synthesis is the pure oligonucleotide attached to the support. In this reaction scheme, the stepwise addition of individual monomers to a single, growing end of a oligonucleotide ensures accurate synthesis of the desired sequence. Moreover, unwanted side reactions are eliminated, such as the condensation of two. oligonucleotides, resulting in high product yields.
In some instances, it is desired that synthetic oligonucleotides have random nucleotide sequences. This result can be accomplished by adding equal proportions of all four nucleotides in the monomer coupling reactions, leading to the random incorporation of all nucleotides and yields a population of oligonucleotides with random sequences. Since all possible combinations of nucleotide sequences are represented within the population, all possible codon triplets will also be represented. If the objective is ultimately to generate random peptide products, this approach has a severe limitation because the random codons synthesized will bias the amino acids incorporated during translation of the DNA by the cell into polypeptides.
The bias is due to the redundancy of the genetic code. There are four nucleotide monomers which leads to sixty-four possible triplet codons. With only twenty amino acids to specify, many of the amino acids are encoded by multiple codons. Therefore, a population of oligonucleotides synthesized by sequential addition of monomers from a random population will not encode peptides whose amino acid sequence represents all possible combinations of the twenty different amino acids in equal proportions. That is, the frequency of amino acids incorporated into polypeptides will be biased toward those amino acids which are specified by multiple codons.
To alleviate amino acid bias due to the redundancy of the genetic code, the oligonucleotides can be synthesized from nucleotide triplets. Here, a triplet coding for each of the twenty amino acids is synthesized from individual monomers. Once synthesized, the triplets are used in the coupling reactions instead of individual monomers. By mixing equal proportions of the triplets, synthesis of oligonucleotides with random codons can be accomplished. However, the cost of synthesis from such triplets far exceeds that of synthesis from individual monomers because triplets are not commercially available.
There thus exists a need for a method to synthesize oligonucleotides with random codons which alleviates genetic redundancy incurred through present synthesis methods using individual monomers and does not have the prohibitive costs associated with methods using presynthesized triplets. The present invention satisfies these needs and provides additional advantages as well.